1. Field of the Invention
The present invention relates to an optical element suitably used for a lens, a filter, a grating, an optical fiber, a planar optical waveguide, and the like. In particular, the present invention relates to an optical element whose refractive index does not change with temperature.
2. Description of Related Art
An optical element such as a lens, a filter, a grating, an optical fiber and a planar optical waveguide is formed of transparent material as an optical element for controlling a mode of light propagation such as transmission, reflection, refraction, and diffraction in modern society. Transparent inorganic materials such as silicic glass materials and metallic oxides are in widespread use as transparent material. Whereas, in recent years, transparent organic polymers with superior moldability, economy, and lightweight properties have also become widespread, and have come into practical use as lenses for spectacles, objective lenses for optical disk, plastic optical fibers, polymer planar optical waveguides, and the like. For example, there is some description in Fumio Ide, “Optoelectronics and Polymeric Material” Kyoritsu Shuppan (1995).
As for the transparent inorganic materials and transparent organic polymers, their refractive indexes, however, change with temperature. Therefore troubles often occur in the case of using them for optical elements generically referred to as lenses for an optical disk device, such as CD and DVD, trying to improve performance of the device by focusing light down to focusing limit (diffraction limit), and optical fibers and planar optical waveguides in which light propagates over a long distance. In particular, as for diffractive optical elements such as Bragg gratings used for optical communications and optical sensors, it is known that performance of the elements is deteriorated significantly because change of the refractive index causes change of the grating constant (optical distance equivalent to grating interval).
Conventionally, various methods have been proposed as a method for making the refractive index of transparent material temperature-independent (athermalizing). For example, in the case of a planar optical waveguide 4 illustrated by FIG. 1D, the following method that the waveguide 4 is constructed in such a way that the thermal expansion coefficient of material constituting an optical waveguide layer 4a and the thermal expansion coefficient (linear expansion coefficient) of material constituting a substrate 4b are opposite in sign, and the waveguide 4 is athermalized by compensating for change of the refractive index of the waveguide layer 4a due to temperature change, is proposed. As examples of the above method, Yasuo Kokubun, “Technology for Making Optical Circuit Independent of Temperature”, Applied Physics, Vol. 66, pp. 934 (1997) and JP-Tokukai-2000-352633A are taken.
Alternatively, when a fiber Bragg grating is used, the method of supporting and packaging the fiber Bragg grating with a material having a thermal expansion coefficient opposite in sign to a silicic material constituting the fiber Bragg grating to compensate for thermal expansion of the silicic material is proposed. For example, A. Sakamoto et al., “IEICE Transactions on Electronics”, Vol. E-83C, pp. 1441 (2000).
As a method for athermalizing transparent material itself instead of compensating by auxiliary material like this for temperature-dependent change of the refractive index of transparent material constituting an optical element, the method of doping transparent material with a material having a rate of change of the refractive index with temperature opposite in sign to the rate of change of the refractive index of transparent material with temperature is known. Methods like this are described in, for example, JP-Tokukai-2001-141945A and JP-Tokukai-2002-020136A.
However, methods like this are limited essentially to materials wherein a silicic glass material is doped with boron oxide as shown in the above-described gazettes. The methods do not make such widespread applications possible, as inorganic transparent materials except silicic glass and transparent materials composed of an organic polymer.
Furthermore methods of obtaining athermalized material by forming mixture or complex composed of two materials whose rates of change of the refractive indexes with temperature are opposite in sign are proposed. As an example of methods like these, the method described in JP-Tokukai-2001-201601A is taken. However as for this method, because of using means that organic material is mixed into inorganic glass material practically, there is a problem that phase separation of the inorganic material and the organic material occurs, and light scattering on phase separation interface undermines optical transparency. Suppression of this problem must rely on sol-gel method in which organic-inorganic complex is formed as starting material and then is heat-treated, as shown in the above-described gazette. In this case, there is a problem that it becomes difficult to apply this method to forming optical elements requiring high dimensional accuracy because of significant change in volume accompanying condensation polymerization in a molding process of optical elements.
As above, various methods are proposed as a method for forming athermalized optical elements. And yet problems remain, such as auxiliary material except transparent material forming an optical element is required, material series forming an optical element is limited, decrease of the transparency accompanying light scattering occurs, and dimensional distortion in a step for forming an optical element occurs.